The present invention relates to a ball screw less to cause breakage or noises, and in particular, to a ball screw of large lead suitably used at high speed sending and high speed rotation.
Machine tools or various kinds of devices have recently been demanded to be worked at high speed, and in company therewith, ball screws used to them have also progressed to the high speed.
For turning to the high speed, there is a large lead, but in a ball screw of the large lead, a forgoing return tube system has been difficult to smoothly rake balls into a return tube placed at upper part of the balls.
This fact will be explained referring to FIG. 6. The same is a view where a shaft 61 of an 20 mm axis diameter and a ball screw having a 40 mm lead is seen from a side of the return tube positioned in front with respect to the drawn paper of FIG. 6. In this ball screw, the return tube (not shown) is arranged such that a rake angle of the ball 62 is 0°. That is, the end part of the return tube (a part having an introducing mouth of the ball 62) is arranged such that the ball 62 coincides with a tangent of the shaft 61 in a raked position.
Thus, if raking the balls 62 placed in the raked position into the return tube, since the balls 62 interfere with a slashed part (see FIG. 6) of a land of the shaft 61, operationability of the ball screw is worsened, or abrasion of the ball or breakage in the land happen.
Therefore, for dissolving inconveniences as mentioned above in the present ball screw of the large lead, the return tube is arranged with taking a large rake angle (that is, the return tube is arranged such that an angle exists between the end part of the return tube and the tangent of the shaft at the raked position) so as to prevent occurrence of said interference.
However, if the return tube is arranged with taking the large rake angle, a change of angle is large in the rolling direction when the balls are raked into the return tube from a raceway of the balls. Then, a force added by the balls colliding to a tang projected at the end of the return tube for raking the balls becomes large, so that the tang is easily broken, and strength in the tang obstructs progression toward the high speed (faculty for turning to the high speed rotation).
Further, if the change of angle is large, the force is made large by the balls colliding to the shaft groove or nut ball groove of the shaft or the land, and another problem is present that the shaft groove of the shaft is easily spoiled.
On the other hand, JP-A-2000-18359 discloses such a ball screw which is not furnished with a tang at the end of the return tube for causing the balls to collide to and raking them into the return tube. Since this ball screw is furnished with a cut section gradually becoming narrower in width at the end of the return tube, the balls are slowly guided to an inner part of the return tube, while being guided by both sides of the cut section.
However, unless forming the cut section in precise shape, there is possibility that the ball screw cannot rake the balls, or the cut section is caused with a bias abrasion in edge parts.
For more raking the balls from the cut section, the shaft groove of the shaft should be shallow, but a shallow nut ball groove is not probably responsible to large load.
Further, in the case of the ball screw of the large lead, it is difficult to form the cut section in precise shape, and a problem is assumed to appear also in cost.
On the other hand, in the nowadays ball screw, a design is generally made on the ball raking part with reference to the path where a center of the ball passes. However, a conventional designing way has tendencies that the change of angle is large in the ball rolling direction when the balls are raked from the ball raceway into the return tube, or a gap (a difference in level) is large between the surface of the nut ball groove and the inner face of the end of the return tube.
In case the ball screw is served at high speed, the balls are rotated while being urged to the surface of the nut ball groove owing to a centrifugal force. Therefore a problem occurs when the balls pass the part of raking the balls, large noises are generated owing to the change of angle or the difference in level.